External monooxygenases

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

All the internal monooxygenases that have so far been purified and characterized contain flavin coenzymes. The external hydrogen donors include reduced NAD, reduced NADP, ascorbic acid and sulfhydryl compounds. Cofactors required for the external monooxygenases are flavin, pteridine, copper, nonheme iron and heme as cytochrome P-450. In some monooxygenase reactions, enzymes and/or electron carrier systems other than monooxygenase itself are involved in the transfer of an electron or hydrogen from the external hydrogen donor to the cofactor involved. 

Two classes of monooxygenases are known. Those requiring a cosubstrate (BH2 of Eq. 18-36) in addition to the substrate to be hydroxylated are known as external monooxygenases. In the other group, the internal monooxygenases, some portion of the substrate being hydroxylated also serves as the cosubstrate. Many internal monooxygenases contain flavin cofactors and are devoid of metal ions. 

Scheme 10.8 Schematic representation of pFtPhPOR adsorbed at the external crystal surface of zeolites (a) and of the methane monooxygenase catalysed cycle (b) (after Nagiev et a/.[77]).

The name of internal monooxygenase was proposed by Hayaishi and Nozaki643 for this type of enzyme those that require a cofactor are called external monooxygenases. 

The latter enzymes had earlier been called mixed function oxidases by Mason in order to characterize their double functions as oxygenases and oxidases. For the reduction of one oxygen atom of molecular oxygen to water two electrons from an external donor have to be supplied. Sometimes the substrate itself can provide these reducing equivalents, so that for these enzymes the term internal monooxygenases may be used ... 

In an alternative approach to mimic tyrosinase activity a copper(I)-copper(n) redox couple and a hydroquinone-quinone redox couple were incorporated in one complex (scheme 17). The hydroquinone moiety should act as an electron shunt between an external reducing agent, i.e. ascorbic acid, zinc or electrochemical reduction, and the copper ions. Catalytic oxygenation by monooxygenases is usually accompanied by the formation of water, with the aid of an external electron and proton source.35 46 Activation of O2 by dinuclear copper(I) complex 58 results in superoxo- or p-peroxo-dicopper(II) complex 59, which oxygenates an external substrate molecule. Internal electron transfer to quinone dicopper(II) complex 60 is followed by quinone to hydroquinone reduction. The electron transfer system shown here is reminiscent of the quinone based systems found in the primary photochemical step of bacterial photosynthesis, and in (metallo)porph3nin-quinone electron transfer systems.In contrast to expectation, the hydroquinone dinuclear copper(II) complex 60 (L = (2-pyridylethyl)formidoyl, scheme 17), designed to mimic step c in this cycle, is a stable system in which the hydroquinone moiety is not oxidized to a quinone structure 61. 

External monooxygenases" require an external electron (or H-atom) donor DH2 for reducing the second 0-atom to water ... 

External monooxygenases can be further subdivided depending on the external reducing agent DH2 required by the enzyme to function. The... 

Other external monooxygenases use reduced iron-sulfur proteins [6], ascorbate [7] or other substrates as DH2 in the hydroxylation of... 

Due to the fact that - in contrast to monooxygenases - no external nicotinamide cofactor is involved in any of the peroxidase cycles, peroxidases are highly attractive for preparative biotransformations. A number of synthetically useful reactions can be achieved (Scheme 2.175) [1312-1314]. 

Cytochromes P450 belong to external monooxygenases. This impUes that they need an external electron donor which transfers the electrons necessary for oxygen activation and the subsequent substrate hydroxylation. Two main classes of cytochromes P450 principally different with respect to their electron-supporting system can be defined ... 

These examples demonstrate the possibility to activate O2 at a single Cu(I) center, yielding a reactive species that can oxidize a substrate without external electron input. This supports the idea that the [Cu02]" adduct in Dj6M (dopamine /i-monooxygenase) and PHM attacks the C-H bond before the electron input of the second copper center. It needs to be pointed out that the supramolecular strategy was mandatory to evidence this behavior. The example... 

Brain tryptophan-S-hydroxylase is a monooxygenase requiring molecular oxygen as a cosubstrate and an external electron donor, tetrahydrobiopterin, the cofactor also required by phenylalanine and tyrosine hydroxylases. 

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